Municipal sewage and almost all industrial wastewater contains organic compounds in one form or other. A portion of these organics is biodegradable, i.e., the action of certain bacteria and microorganisms can decompose these organics into non-harmful compounds, namely carbon dioxide and water. In fact, the microorganisms utilize these organic compounds as food and thus, the organics are essential for the growth and reproduction of bacteria and other microorganisms. These microorganisms are aerobic in nature and oxygen is needed for survival as well as their growth.
This natural phenomenon is utilized to effect the removal of biodegradable compounds (commonly referred to as BOD) from the wastewater before it is discharged into public water streams where, if not removed, these organics would cause depletion of dissolved oxygen, a condition which would be harmful to the aquatic environment. The activated sludge process employs the above concept and is the most commonly used biological treatment method.
In conventional activated sludge processes the wastewater is aerated in a large aeration tank. For the removal of organic compounds, the aeration tank must have an adequate supply of oxygen (or air) and certain microorganisms (aerobic bacteria, protozoa). Other requirements are adequate amounts of micro-nutrients, absence of compounds toxic to bacteria and a certain dissolved oxygen level. In a nutshell, in the aeration tank, environmental conditions are maintained so as to promote the optimum growth of microorganisms and, thus, achieve maximum BOD removal.
The biological floc is suspended in the aeration tank and the tank contents are in turbulent regime to maintain the suspension. The wastewater containing suspended and dissolved organics is introduced at the inlet end where it is mixed with the returned sludge and is discharged into the tank. The tank contents, including wastewater, returned sludge and suspended biological floc are known as mixed liquor. The mixed liquor is continuously withdrawn from the tank and retention time in the tank is varied to achieve the desired treatment efficiency.
The mixed liquor is discharged into a clarifier/setting basin where the biological solids settle down to form sludge, and the clarified water overflows over the effluent weir. A portion of the sludge is returned to the aeration tank to maintain a steady concentration of BOD removing microorganisms in the aeration tank. Any excess sludge is wasted.
In conventional activated sludge, air is supplied to the tank by surface aerators or submerged diffusers. As the air contains only 20% oxygen, an enormous amount of air has to be supplied to satisfy the oxygen requirements. The primary disadvantages of an air system are large tank volumes required, lower oxygen transfer efficiency, and high oxygen consumption (per lb. of BOD removed).
In the past few years, there has been increased acceptance of pure oxygen activated sludge process. In this process, an oxygen rich gas (90%-95%) is supplied to the aeration tank rather than air. The primary advantages are improved oxygen transfer efficiency, better sludge settleability, less land area is required, lower capital and operating costs, and lower oxygen consumption (per lb. of BOD removed).
The effluent from the clarifier has some BOD, suspended solids and bacteria and it has to be disinfected ("bacteria kill") prior to discharging into the receiving stream. Chlorination is the most common mode of disinfection, however, the recent trend is towards the use of ozone for disinfection because (1) chlorination is known to produce toxic and carcinogenic compounds (chlorinated hydrocarbons), and (2) chlorine handling is hazardous.
The supply for the pure oxygen system consists of two components, the oxygen plant and the oxygen diffusion or transfer device. Except in very small plants, the oxygen has to be generated at site for improved economics. In the oxygen plant, air is compressed, liquified and separated into oxygen and nitrogen in a distillation column. The oxygen generated usually has a purity of 90% to 95%.
The next step is the transport and transfer of oxygen into the mixed liquor. Two methods of oxygen transfer are presently in the market. One "closed tank" system relies on a surface aerator or similar equipment to transfer oxygen from a closed ullage space to the mixed liquor. The other, called "open tank" system, employs a submerged (in mixed liquor) rotating diffuser or similar equipment to produce and disperse finer oxygen bubbles. Both of these systems achieve over 90% oxygen transfer efficiency.
Ozone, for the ozone disinfection system, although required in very small quantities (ozone dosage varying from 3-10 mg/l), is generated on site as it is naturally unstable and decomposes to oxygen over a period of time. The ozone system has essentially three components, (a) ozone generation; (b) ozone transport; and (c) ozone transfer to wastewater.
In almost all the commercial installations, ozone is generated by passing oxygen through a controlled corona discharge. Alternating current corona discharge is produced across two glass dielectric electrodes at potentials between 7,500-50,000 volts and 50-2,000 HZ frequency. The amount of ozone formed is directly proportional to the power dissipated in the discharge. However, 85%-95% of applied electrical energy is dissipated as heat in the discharge space. As ozone decomposes more readily at higher temperatures, this heat must be removed. Thus, all ozone generators are either air or water cooled.
Both air or oxygen can be used as feed gas and only a small amount of the feed gas is converted to ozone. In most of the commercially available generators, the conversion averages 1% for air feed and 2% for oxygen feed. With pure oxygen feed, the most efficient conversion occurs at 1.5% to 2% (least power consumed per lb. of ozone produced), however, the commercial generators can produce 4% ozone or more.
The efficiency of an ozone generator is also affected by the oxygen feed gas purity. Presence of nitrogen, carbon dioxide, etc., in the feed stream results in the slightly higher power consumption.
As the economical conversion levels are low (1.5%-2%) a large amount of carrier gas has to be used to produce the required dosage of ozone.
In a municipal sewage treatment plant, oxygen (rather than air) is invariably used to produce ozone for the reasons outlined below:
(a) The cost of oxygen production is offset by the increased conversion obtained in the ozone generator.
(b) The ozonation off-gas containing large amounts of oxygen (up to 90%) can be reused to supply the oxygen requirement of the biological treatment.
(c) As a side benefit the dissolved oxygen level of effluent is increased to meet the residual oxygen demand created by effluent BOD and COD, thus, eliminating post aeration.
Economics is foremost in the mind of all the designers and planners. It is obvious that pure oxygen shall be used for the ozone generation as higher ozone conversion is obtained, reducing the carrier gas requirement (to less than half).
High capital and operating costs associated with oxygen generation makes it logical to combine the ozone disinfection process with the pure oxygen biological treatment step. The ozone tank off-gas contains a significant amount of oxygen (up to 90%), and considering the high cost of oxygen production, it is imperative that ways be found to use this off-gas. It has been recognized by prior researchers and a few methods exist for the reuse or recycle of the off-gas.
The process proposed in this patent application utilizes a different method for the efficient utilization of the ozone tank off-gas and has significant advantages over the earlier processes.
The ozone in its oxygen carrier gas is normally introduced near the bottom of the tank using porous diffusers. Ozone is dissolved and consumed in the disinfection process. A small portion of oxygen (and other impurities) in the carrier gas is also dissolved in the water and leaves the system with the effluent. However, a major portion of the gas remains undissolved and collects in the space above the water. During its rise through the ozone tank, the carrier gas "strips" substantial quantities of nitrogen and carbon dioxide from the wastewater (upstream of the ozone tank, wastewater absorbs nitrogen on contact with atmosphere and CO.sub.2 as it is released in the oxygenation basin). The stripping action is due to the fact that the oxygen enriched carrier gas has a relatively small amount of nitrogen, CO.sub.2, etc., thus, the low partial pressure causes the transfer of these gases (N.sub.2, CO.sub.2) from the liquid to the gaseous phase. Therefore, the oxygen content of the off-gas is slightly lower than the feed gas.
As the ozone disinfection step is almost invariably accompanied by the pure oxygen activated sludge process, one of the simplest ways to utilize the off-gas would be to divert the oxygen rich off-gas to supply the requirements of the aerobic treatment process. An example of this system is McWhirther et al. U.S. Pat. No. 3,660,277. In this process the only external gas supplied is the oxygen feed to the ozone generator. It effectively uses the ozone off-gas and maintains high oxygen purity in the feed gas to the activated sludge plant. However, as the oxygen requirement of the ozonation step in a municipal sewage treatment plant is normally greater than the oxygen required in the activated sludge process, a large amount of expensive oxygen enriched gas has to be wasted. Thus, for the most commonly occurring municipal sewage, the McWhirther patent does not eliminate the oxygen wastage, it merely reduces the amount of wastage. For convenience, the McWhirther process hereinafter has been referred to as "zero recycle system".
Another alternative is to recycle the off-gas to the ozone generator for the production of more ozone. However, each time the oxygen enriched gas is recycled and then fed to ozone contact tank, it picks up nitrogen and carbon dioxide in the ozone contact tank due to the stripping action as described above. Thus, the impurities level buildup with each recycle and the buildup continues until the partial pressure of nitrogen and carbon dioxide in the feed gas is in equilibrium with the dissolved gas level in the wastewater.
Thus, if the contaminants are not removed, the oxygen content of the feed gas to the ozone generator decreases sharply, resulting in lower efficiency and increased power consumption in the ozone generation step.
It is apparent that the buildup of impurities should be limited to a reasonable level to achieve the most economical operation. This problem has been recognized by few researchers and few methods have been proposed.
Kirk U.S. Pat. No. 3,945,918 proposed a complete recycle system in which all the ozone off-gas is recycled to the ozone generator. To limit the buildup of contaminants, a provision is made to vent (purge) a portion of the recycle gas and introduce fresh make-up oxygen at the inlet to the ozone generator. However, to achieve economical oxygen purity level in the combined feed to the generator, a significant amount of recycle gas will have to be vented (wasted). Lapidot U.S. Pat. No. 3,732,163 has tried to eliminate this expensive venting (purging) of recycle gas by removing the contaminants by absorption of impurities in a cooled liquid. In the event the absorption step is not sufficient to increase oxygen purity to an economical value, a bleed valve has been provided to vent a portion of the gas.
Both of the above expedients are expensive and have no known commercial installation in existence or in design phase. For convenience, the above two processes hereinafter will be referred to as "total recycle system."
Key et al. U.S. Pat. No. 4,132,637 has dealt with this dual problem of eliminating oxygen wastage and limiting impurity buildup, two seemingly conconcurrent phenomena, in the "partial recycle system". In this process, a substantial portion of the off-gas is recycled and the rest of the off-gas is diverted to the oxygenation basin. Fresh oxygen is introduced at the inlet of both the ozone generator and the oxygenation basin to supply the remaining requirement of ozonation as well as carbonaceous BOD removal process. The performance and economics of this process are greatly affected by the recycle rate (to the ozone generator). The recommended feasible range of recycle rate is 30% to 90% and 60% recycle is recommended as optimal, by the inventors, The important and critical features of this process are:
(a) As the recycle rate increases, the impurity level in the feed gas to the ozone generator increases indicating higher power consumption. However, at higher recycle rates, the possibility of oxygen wastage diminishes.
(b) At lower recycle rates the oxygen purity (to ozone generator) increases, however, the chances of oxygen wastage also increase. As described in detail later in this application, some venting of oxygen will occur at the recommended recycle rate of 60% under some operating conditions occurring in municipal wastewater treatment, particularly in the treatment of effluent having low BOD levels combined with high ozone dosage in the ozone treatment. The BOD of primary effluent in a municipal wastewater treatment plant normally varies from 60-300 mg/l. Later discussion herein will illustrate that the partial recycle process may result in oxygen wastage if primary effluent BOD is 190 mg/l or less.